Map of the Earth indicating boundaries, mostly the
midocean ridges, where plates separate (green), and boundaries where plates
converge (heavy red, blue, and black lines). The Kermadec-Tonga Arc is
one of the longest interoceanic arcs on the planet. Submarine Ring of Fire
cruises in 2003 and 2004 visited the Mariana Arc. Click image for larger
view and more details.

Exploring for Hydrothermal Systems Along a Submarine
Volcanic Arc

Edward T. Baker
Supervisory Oceanographer
NOAA Vents Program

Submarine Volcanic Arcs

Volcanic arcs are curved chains of volcanoes separated from
the adjoining ocean basin by a deep submarine trench. They mark boundaries
where the Earth's tectonic plates are consumed by sinking, just as midocean
ridges mark boundaries where plates are created. Arcs are also the surface
expression of new magma formed by the plate sinking, as this hot magma
rises and forms volcanoes. A good example of this process is Mt. St. Helens
in Washington State, where recent eruptions remind us that this volcanic
activity is a powerful force. The Kermadec-Tonga Arc is an intraoceanic
arc, one that has oceanic crust on either side, in which case the volcanoes
are mainly submarine. There are almost 22,000 kilometers (~12,000 miles)
of volcanic arcs along the ocean floor, almost all in the western Pacific
Ocean.

Several cruises over the past six years have carefully studied
more than 100 submarine volcanoes, more than 40 of which have been hydrothermal
active. At these volcanoes, cold seawater is heated by the cooling magma
within and beneath them, becomes buoyant, and rises to the volcano summit.
The hot fluid rises like a hot-air balloon into the cold ocean, cooling
as it mixes with the cold seawater. Mixing continuously dilutes the hot
discharge and increases its volume as it rises, until the hydrothermal
plume achieves neutral buoyancy and is dispersed by the local currents.
This hydrothermal circulation pattern occurs wherever seafloor cracks can
channel seawater to layers of hot rock.

The Tonga-Kermadec Arc

The Tonga-Kermadec Arc is part of the Pacific Submarine
Ring of Fire, marking the location where the Pacific Plate sinks
beneath the Australian Plate. The arc stretches for some 2500 km from New
Zealand to Samoa, and is thought to include over 100 submarine volcanoes.
Four cruises since 1999 have systematically studied these volcanoes for
hydrothermal sophisticated instrument package that can detect even the
slightest scent of a hydrothermal plume. If hydrothermal indicators
are found, further surveys are conducted to try and pinpoint the location
of the fluid sources on the volcano. These surveys are used to guide the
later seafloor studies, such as the PISCES submersible dives to be conducted
this year.

Location
map for the Kermadec-Tonga Arc, showing the study areas covered
by four cruises since 1999. The New Zealand American Submarine
Ring of Fire 2005 cruise will visit submarine volcanoes in the
regions explored by the NZAPLUME 1-3 cruises. Click image for
larger view and more details.

A
closeup of the primary tool used to map hydrothermal plumes. A
ring of plastic sampling bottles surrounds the CTD, an electronic
package that monitors seawater Conductivity, Temperature, and Depth.
The bottles are closed on command from the ship, usually when a
scientist monitoring the sensors sees strong evidence of a plume.
The CTD carries other sensors that measure optical light backscattering,
seawater pH and Eh, and other chemical species. Pictured here are
Ed Baker (left) and Cornel deRonde (right). Click image for
larger view and more details.

Conducting a series of cruises along the arc allows scientists
to piece together a view of the distribution of hydrothermal activity on
the arc. Results of the 1999 (NZAPLUME 1) and 2002 (NZAPLUME 2) cruises
along the southern Kermadec-Tonga arc, for example, found hydrothermal
plumes over at least 17 submarine volcanoes. Because of the different water
depths of the volcano summits, injection of hydrothermal material may occur
anywhere from 1800 meters (~1 nautical mile) to <100 meters (~325 feet)
deep. Hydrothermal fluids discharged directly into the shallow, photosynthetically
active layer of the ocean might fertilize plankton growth, increasing the
ocean productivity at those sites.

Cross-section
along the southern Kermadec-Tonga Arc. Each peak is a separate volcano,
formed by magma generated by heating the Pacific Plate as it sinks
into the Earth below the arc. This same effect creates terrestrial
volcano chains, such as the Cascades and the Andes. This plot illustrates
the distribution of hydrothermal plumes by plotting the relative concentration
of very fine-grained particles... Click image for larger view and
more details.

Exploring the Ocean, Expanding Our Knowledge

Hydrothermal venting can develop on only a minuscule portion
of the seafloor, yet its effects influence the entire ocean and support
a unique biological community that may be Earth's earliest ecological niche.
While we understand many of the basic principles that govern this phenomenon,
we remain deeply in the explorative stage of this research. We are surprised
almost daily by discoveries of vents where we expected none, chemical compositions
that were unpredicted, and the cataloging of new biological species. The
ocean is a harsh and unyielding environment, and new progress is hard won.
But the excitement and value of increasing our knowledge of one of Earth's
basic processes makes this exploration among the most rewarding we can
imagine.

Many of the submarine volcanoes in the Kermadec Arc are large calderas. Calderas are huge craters that are created when so much
material is expelled from a volcano during a large explosive eruption that
the top of the volcano collapses back on itself. Crater Lake in Oregon
is an example of a volcanic caldera on land that formed during a huge eruption
about 7000 years ago. Thus the existence of large submarine calderas in
the Kermadec Arc is clear evidence that these volcanoes have a very violent
past.